Clamshells andClimate Change

What seal bones and clamshells teach us about past climate

Archaeology

Catherine West was having no luck. Knee-deep in the cold waters of Dutch Harbor, Alaska, West scanned the rocky seabed for butter clams. The clams had buried themselves in the sand, as clams are wont to do, so she was looking for the telltale siphon—a small tube they stick out, to suck up the nutrient- and oxygen-rich seawater.

“It looks like a black straw,” called her colleague, geologist Fred Andrus, digging on shore.

West stared doubtfully down at the water, a mosaic of sea stars and spiny urchins under the surface. “Everything looks like a black straw,” she said.

Catherine West

“I have tremendous respect for the people who survived here for 9,000 years and continue to live here,” says West. “I’ve often thought, ‘Would I survive?’ And the answer is probably no.”

Aleutian Islands, Alaska

West, a Boston University research assistant professor of archaeology, had come to the remote island of Unalaska—800 miles southwest of Anchorage in the Aleutian Islands—for a week in August 2017 to solve a mystery whose answers date back thousands of years. The butter clams, if the scientists could find some, were to be a critical piece of evidence.

Scientists know, through sediment and ice core samples, that the Northern Hemisphere underwent a dramatic cooling period between about 2,500 and 4,700 years ago. And when archaeologists found a surprising cluster of bones in an ancient garbage heap here, they suspected that this global climate change may have had a powerful effect on the island and its inhabitants. Using two sets of artifacts, seal bones and clamshells, as well as some other clues, West and her colleagues want to find out how far the temperature dropped in Unalaska during that cold snap and—if the change were drastic—how did the people and the animals adapt?

The answers to these questions matter. The discoveries that West’s team hopes to make will teach us a lot about present-day commercial fisheries, marine mammal behavior, and global weather patterns, as well as the impact of climate shifts on all these things. Dramatic effects of climate change are already assailing the Arctic—like the rapidly thawing permafrost throughout Alaska—and scientists are beginning to understand better how fluctuating conditions in the far north influence weather around the globe.

Unalaska Island

Between about 2,500 and 4,700 years ago, the Northern Hemisphere cooled dramatically. Was it cold enough for sea ice here?

“We’re intimately connected on a climate level,” says West. “In Boston a few years ago, when we had feet and feet of snow, we know that that was an Arctic system coming down.”

“Up here, they are already feeling the big influences of warming, especially further north. Their villages are crumbling into the ocean,” she continues. “We should be watching them as a predictor of what we are going to experience in Boston Harbor and New England. But I’m not sure that most people are as aware of that as they should be.”

Whipsawing williwaws

Unalaska Island is a strikingly beautiful place. Craggy mountains, softened by a coating of deep velvety green turf, rise at every turn, ringing cobbled beaches and harbors. The best known of these is Dutch Harbor, immortalized in the Discovery Channel series Deadliest Catch, which follows king crab fishing crews through winters on the Bering Sea. In “Dutch,” as everyone calls it, and throughout the island, humans are dwarfed by the magnificent green hills, as in some kind of Hobbit-y alpine fairyland.

But the most striking characteristic of the island, one impossible to capture in photographs, is the ceaseless wind. People call the Aleutians the birthplace of the winds, and on this remote chain of about 69 islands, stretching for 1,100 miles from the tip of the Alaska Peninsula along the edge of the Bering Sea, the winds live, die, and reincarnate in the same instant—whirling from all directions. On Unalaska, the wind has its own name: williwaw. The williwaw whips down from the green hills, coats cars with gray silt, and topples cargo containers that aren’t sufficiently weighed down with rocks. Sometimes it just ruffles your hair. But it is always there.

“I find it thrilling. I find it slightly scary, and also incredibly beautiful and raw,” says West of the island. “It still has that wild feel about it, and you are so close to the environment everywhere you go.”

The inhabitants of the Aleutian Islands, known as Aleuts or the Unangan people, have lived there for 9,000 years, enduring extreme isolation, volcanic eruptions, earthquakes, tsunamis, frequent fog, storms, rain, snow, and gale-force winds. Maritime hunters and gatherers, they had the sophistication and knowledge to live off the land and survive calamity. A volcano would erupt, leaving a layer of ash, and the Unangan people would come right back and build on top of it. “Aleuts lived in the most dangerous, even catastrophic northern area,” write archaeologists Allen McCartney and Douglas Veltre in the journal World Archaeology. “Northern Alaska, the Canadian Archipelago, and northern Greenland are, by comparison, gentle places of predictable seasonal changes.”

The US government evacuated the Unangan people to the mainland during World War II, after Japanese bombers attacked the American air base at Dutch Harbor. Though some Unangan people returned to Unalaska, many did not; but we know from oral history, artifacts, and archaeology how the ancient people managed to thrive on the island. With no trees, they built houses of sod and made cooking fires from seal oil and driftwood. They crafted sewing needles from albatross bones and sewed waterproof parkas from long ribbons of seal gut. One plentiful thing was food: Pacific cod, sea lions, whales, otters, and clams. Archaeologists, studying buried Unangan settlements and artifacts, note the remarkable stability and persistence of their culture.

There is, however, one mysterious blip, during that global cooling period known as the Neoglacial. The first hint of this blip came in the late 1990s, when archaeologists excavated a site in Unalaska called Margaret Bay and found, surprisingly, the bones of a ringed seal and possibly a polar bear. Then, in the early 2000s, the Alaska department of transportation decided to build a bridge on the island. The bridge footings were to land on the buried remains of a village, so archaeologists excavated that site as well.

Digging a midden

Archaeologists excavate the Amaknak Bridge site. The bones they found include ice-loving mammals like ringed seals.

Photo by Richard A. Knecht, courtesy of the Museum of the Aleutians.

When archaeologists examined the Amaknak Bridge site artifacts, they found the usual bones—harbor seals, sea lions, fur seals, otters, puffins, murres, cod, and salmon. But they also found more unexpected artifacts: abundant remains of bearded and ringed seals, including juvenile ringed seals.

The finds were surprising, since both species spend critical parts of their lives on sea ice; ringed seals, for instance, birth their pups on ice, hiding them in layers of snow to protect them from polar bears. But there’s no sea ice on this island today, or in historical memory, even in the dead of winter. In 2017, the Arctic sea ice crept only as far south as Alaska’s Bristol Bay, still about 350 miles north of Unalaska. That’s further than the distance from Montreal to Boston.

“I remember thinking, that’s crazy,” says West, recalling the first time she heard about the find. “But I know the person who was identifying the stuff, and I have great confidence in her work, and I thought, that must be a pretty remarkable place.”

So, what were these ice-loving seals doing in a place with no ice? Did they get lost? Did they drastically change their behavior? Did the Unangan hunters start traveling far north to harvest them? Or did the Neoglacial cooling period hit the Aleutians so hard that the harbor iced over? That would mark a dramatic change for the island’s inhabitants.

“It would be like Boston Harbor freezing over and polar bears appearing on our landscape,” says West. “That would be kind of shocking. We’d have to change the way we do things a little bit.”

The artifacts from the Amaknak Bridge and Margaret Bay sites, along with those from another nearby site, Amaknak Spit, now live at the Museum of the Aleutians in Unalaska. They are a remarkable scientific collection. The most valuable bounty comes from the site’s garbage piles, or middens, where thousands of shells of butter clams have leached calcium carbonate into the acidic soil, keeping bones and teeth remarkably well preserved, with plenty of shells left as well.

Working with a $650,000 grant from the National Science Foundation, West and her two co-PIs—Michael Etnier, an anthropology research associate at Western Washington University, and Fred Andrus, a professor of geology at the University of Alabama—plan to reexamine the animal bones, searching for more evidence of bearded and ringed seals, as well as evidence of other ice-loving mammals like polar bears and belugas. Her team will also chemically analyze both ancient and fresh clamshells, trying to pin down an estimate of ancient water temperature. The team hopes that these two lines of evidence, along with an examination of other bones and artifacts, will paint a more detailed picture of what happened here thousands of years ago.

Cracked teeth and flipper bones

Michael Etnier picks up a heavy-duty Ziploc bag and pours a pile of bones onto a yellow plastic tray. As West and her two students, Anna Goldfield (GRS’17,’17) and Carly Buta (CAS’17), look on intently, Etnier begins a rapid-fire identification: sea lion distal radius, fur seal scapula, sea lion fifth toe, cracked canine tooth. Etnier, a marine mammal expert, is overseeing the mammal bone identification, and his skill and speed are astonishing; to the untrained eye, it all looks like garbage.

Michael Etnier

“The kinds of questions that we’re able to ask about these bones right now weren’t even dreamed of 20 years ago,” says Etnier.

“It is garbage!” Etnier shouts gleefully, sorting a harbor seal flipper bone from a fur seal humerus. “Somebody ate this as a meal 3,000 years ago and tossed it in the heap.”

“I love the bones because I can pick one up and tell you, ‘Oh, that’s a Pacific cod,’ and I know someone here was eating Pacific cod for dinner one night,” says West. “And it sounds silly, but it feels powerful.”

Etnier picks up a graceful curve of bone studded with tiny teeth—a seal mandible. “To figure out if these ringed seals were on the ice edge 30 miles from here or were being brought in, we need a massive, massive sample—which is what makes the Amaknak Bridge site such an amazing resource,” he says. “It was a gigantic site; they saved everything; it’s all been really well documented. So we’ll have these gigantic samples and we can then dig deeper and deeper into some of these research questions.”

The Bering Sea provides a critical habitat for many marine mammal species—not just residents, like sea otters, but also migrants, like right whales and orcas. Because each plays a role in a complex ecosystem that reaches southward to the Pacific Ocean, understanding how these mammals may—or may not—adapt to climate change could have widespread implications. “In the Bering Sea and North Pacific Ocean, we have seen marine mammals responding to environmental changes both through their behavior and in the health of their populations,” says West. For instance, in the last five years, an unusual number of orcas have been plundering fishing lines in the Bering Sea, wreaking havoc on the halibut industry. As the mammals change, says West, “their critical relationship to the ecosystem—and to commercial interests—becomes increasingly clear.”

Etnier’s analysis may offer more clues about mammals’ ability to adapt and also about the extent of past climate change: if he finds bones that were cracked to remove fat and marrow, for instance, that might indicate that people were short of food, struggling with the cold weather. But despite the promise hidden in this pile of bones, Etnier says that they alone can’t answer the climate question. That requires shell chemistry, he says: “The humble clam.”

The humble clam

Fred Andrus is back on the Amaknak Spit, having found no butter clams the previous day. He carries a spade and does not look optimistic. “If I were betting,” he says, “I’d bet against me.”

Andrus walks to the waterline and sticks his spade in. He breaks through a layer of black rock and popweed, finds his way to gravelly sand, and digs in. About six or eight inches down, water begins to fill the hole. “Holler if you see something that looks like a clam!” he says.

Fred Andrus

“What are the rhythms of life of each individual organism, and how does that impact the chemistry?” asks Andrus.

Andrus is an expert in sclerochronology—studying the growth of shells, corals, and other hard parts of soft creatures, to better understand the conditions in which they grow. Clams are like flight data recorders, Andrus likes to say; “the trouble is, they’re not in a language we can understand.”

West, Goldfield, and Buta had spent days at the museum lab sorting through hundreds of ancient clamshells, selecting specimens for Andrus to analyze. (“It felt like a hundred billion clams,” says Goldfield, with a laugh.) Butter clams, which live up to 20 years, build their shells from calcium carbonate at regular intervals, resulting in growth lines that are analogous to tree rings. “Clams are really sensitive to their environment,” says Andrus. “They make their skeleton pretty much directly out of the raw material in the water.” Because of this, the makeup of a clamshell holds data about water chemistry and temperature. Andrus plans to analyze ancient clams in the hope of finding the prehistoric water temperature—and, thus, evidence of sea ice.

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But to analyze the old clams, he needs to create a model, with modern data. Andrus and his graduate student, Christine Bassett, with the help of Missy Good from the University of Alaska Fairbanks, had been collecting temperature readings and water samples in this section of the spit for a year, and now they need the final bit of data, samples from fresh clamshells. Bassett and Good had been diving in the frigid deep water and had found a few baby clams, but Andrus wanted more.

Standing over his waterlogged pit, Andrus saw a few tiny bubbles breaking the surface of the cloudy water. “Maybe?” he says. “Could just be a worm.” But within a minute, he reached in and pulled out a butter clam. He held the gritty clam, a grin spreading across his rain-splashed face. “Awesome,” he says. The clam spit a small burst of seawater.

Digging for clams

“Butter clams are big and tough,” says Andrus. “There is no soft sand. They’re constantly being hunted. They have a rough life, and so they’re good at hiding.”

Butter clams

The data Andrus develops could be useful well beyond this current project, he says. “We’re focusing on butter clams for the archaeologists, but yesterday we collected, like, five different species of shellfish,” he says. Blue mussels, he notes, are a common food source throughout the world. “The mussels have commercial value, and there are actually some significant questions about their shell chemistry, their growth, that’s valuable in itself.”

Regarding the Unalaska project, Andrus cautions that he won’t be able to create a “flawless digital thermometer of the past. We’re never going to be able to come up with a definitive sea surface temperature, but we’ll be able to say it’s cooler or warmer than usual,” he says. “If we use 10 different methods, and they all tell the same story, that’s a pretty compelling story.”

Lessons in resilience

The story of the Unangan people is embedded in the artifacts they left behind, many of which remain buried around the island. West, Goldfield, and Buta drove out to Summer Bay, an inlet of the Bering Sea, to have a look at a recent dig. The bay joins a freshwater lake and the salmon were running home—so many you could see them jumping from the water. The professor and her students walked across the beach to an eroded hillside, where West pointed her trowel at a thin line of black running horizontally across the brown dirt. It’s the earthen floor of a house, with charcoal, shell, and fish bones ground into it, packed down and densely compressed. “To actually stand here and look reminds you—it reminds me, at least—that these were real people,” she says. “I’m looking at other humans’ stuff, and not just doing lab work and counting bones and looking at chemistry.”

What would those resilient ancient people make of us modern folk? In the past few months, our cities have been scorched by wildfires, flattened by hurricanes, shattered by earthquakes. It’s not so easy for us to go back and rebuild, to sew our own clothes, live off the sea. But maybe we can learn some lessons from the Unangan people about the value of listening to the land, living close to it, and adapting.

“The people here were living in an environment that presented dramatic seasonal challenges,” says West. But they were “pretty flexible in the face of change.

“While it may be difficult to see how such a small society can teach us about adapting to climate change today, working at this scale reminds us to look at our own, local environment—how is it changing as the climate changes? What will we need to do to adapt to those changes or protect our own infrastructure and resources?” she adds. “Many past ecosystems and people—like the Unangan people—successfully adapted as their environments changed, so knowing how and why they did will be a critical lesson for us, too.”

What the Clams Say

By Barbara Moran | Video By Devin Hahn

For data on ancient changes in water temperatures, West turns to her colleague Fred Andrus at the University of Alabama, and he, in turn, reaches for the butter clams. Andrus’ lab will chemically analyze the shells of butter clams from Unalaska, harvested and eaten as long ago as four thousand years, to reveal ancient water temperature. This analysis will provide added evidence for (or against) the presence of sea ice.

“We have ambiguous evidence for sea ice,” says Andrus, who has previously analyzed shells from middens in Peru, Belize, and the US Atlantic and Gulf coasts. “So they enlisted me to see if there’s a quantitative measure.”

Butter clams, which live up to 20 years, build their shells from molecular building blocks drawn directly from seawater; as a result, the shells hold data about the conditions in which they grew.

A section of butter clam

Scientists use a compound called Mutvei’s solution to etch away layers of calcium carbonate, and stain the shell a brilliant blue. This technique allows them to measure how much a clam grew each day, over the course of a year. A shorter growing season could indicate a dramatic climate shift.

“When you have a clam that’s happy, living underwater, it’s open and it extends what’s called the mantle, which looks kind of like a little meaty, gooey, gross part,” says Christine Bassett, a PhD student in Andrus’ lab who is conducting the analysis with help from BU students Madeline Duppenthaler (GRS’19), Amanda Garza (CAS’17), and Francis Smith (CAS’18). “And when that mantle is outside of the shell, it’s putting down this calcium carbonate and it’s growing. And when the shell is stressed out, it closes and it retracts its mantle.”

The exact chemical makeup of the calcium carbonate layer—specifically, the ratio of heavier oxygen-18 to lighter oxygen-16 isotopes—is dependent on two things: the temperature and chemistry of the water. Assuming that the water chemistry hasn’t changed dramatically, oxygen ratios should allow the team to see if there were any drastic changes in the water temperature. “Theoretically, the ratios should stay stable,” says Smith, a BU undergraduate biology major. “But colder water will cause the clams to sequester more oxygen-18 into their shells.”

“Basically, the less oxygen-18, the hotter the water,” says Andrus. “But here’s the giant if—if the water chemistry remains constant. We expect the water chemistry will be different in the past, but this gives us a quantitative starting point.”

“If we find that we can tell temperature within a few degrees, and we can measure the growing season within a month, that would probably be meaningful because if sea ice is coming here, that’s a non-trivial climate change,” he adds.

In Andrus’ lab in Alabama, Smith drills tiny pits into the rings of butter clams, then runs the samples of shell dust through a mass spectrometer to collect data on the oxygen isotope ratios. The team will compare these data to similar data collected from modern clams before making any conclusions.

“We have to have a really solid understanding of how that clam is responding to its environmental conditions,” says Bassett. “We need to feel really confident about that in the modern clams, which can only come from observations, before we can apply that relationship back to the archaeological samples.”